Systems and methods for high rate OFDM communications using first and second cyclic prefix lengths

ABSTRACT

Messages transmitted between a receiver and a transmitter are used to maximize a communication data rate. In particular, a multicarrier modulation system uses messages that are sent from the receiver to the transmitter to exchange one or more sets of optimized communication parameters. The transmitter then stores these communication parameters and when transmitting to that particular receiver, the transmitter utilizes the stored parameters in an effort to maximize the data rate to that receiver. Likewise, when the receiver receives packets from that particular transmitter, the receiver can utilize the stored communication parameters for reception.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No.12/966,246, filed Dec. 13, 2010, which is a continuation of U.S.application Ser. No. 12/642,495, filed Dec. 18, 2009, now U.S. Pat. No.7,916,625, which is a continuation of U.S. application Ser. No.12/419,166, filed Apr. 6, 2009, now U.S. Pat. No. 7,804,765, which is acontinuation of U.S. application Ser. No. 10/382,921, filed Mar. 7,2003, now U.S. Pat. No. 7,522,514, which claims the benefit of andpriority under 35 U.S.C. §119(e) to U.S. patent application Ser. No.60/363,218, filed Mar. 8, 2002, entitled “High Rate OFDM CommunicationSystem and Method for Wireless LAN,” each of which are incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The systems and methods of this invention generally relate tocommunication systems. In particular, the systems and methods of thisinvention relate to Orthogonal Frequency Division Multiplexing (OFDM)communication systems, methods and protocols.

2. Description of Related Art

The IEEE 802.11a and 802.11 g standards for wireless LANs, which areincorporated herein by reference in their entirety, herein afterreferred to as 803.11a/g, specify wireless local area networkcommunication systems in the 5 GHz and 2.4 GHz bands. These standardsspecify the use of OFDM as the modulation method used for communication.OFDM is a multicarrier modulation scheme that performs well in wirelesscommunication channels. The 802.11a/g standards provide data rates of 6,9, 12, 18, 24, 36, 48 and 54 Mbps. Different data rates are achieved bytransmitting different, but constant, numbers of bits on all carriers inthe multicarrier system and by operating at different coding rates.Table 1 below illustrates the coding rate and bits per subcarrier foreach data rate for an exemplary 802.11a/g transceiver.

TABLE 1 DATARATE Bits per Subcarrier (Mbps) Coding Rate (R) (N_BPSC) 6 ½1 9 ¾ 1 12 ½ 2 18 ¾ 2 24 ½ 4 36 ¾ 4 48 ⅔ 6 54 ¾ 6

In order to determine the appropriate transmission data rate, the802.11a/g transmitter uses a trial and error method of transmitting atvarious data rates, starting with, for example, the highest or lastsuccessful transmission rate, and waits for a positive acknowledgementindication from the receiver that the packet was successfully received.This simple positive acknowledgment indication method is used tooptimize communications in conventional 802.11a based wireless systems.

SUMMARY OF THE INVENTION

The exemplary systems and methods of this invention use messagestransmitted between a receiver and a transmitter to maximize thecommunication data rate. In particular, and in accordance with anexemplary embodiment of this invention, a multicarrier modulation systemuses messages that are sent from the receiver to the transmitter toexchange optimized communication parameters. The transmitter then storesthese communication parameters and when transmitting to that particularreceiver, the transmitter utilizes the stored parameters in an effort tomaximize the data rate to that receiver. Likewise, when the receiverreceives packets from that particular transmitter, the receiver canutilize the stored communication parameters for reception.

Accordingly, aspects of the invention relate to multicarrier modulationcommunication systems.

Additional aspects of the invention relate to wired or wirelessmulticarrier modulation communication systems that transmit messagesbetween transceivers.

Additional aspects of the invention relate to transmitting messagesbetween a plurality of transceivers in an effort to optimize a datacommunication rate.

Further aspects of the invention relate to exchanging optimizedcommunication parameters between a plurality of receivers in amulticarrier modulation system.

Additional aspects of the invention relate to exchanging communicationparameters between a plurality of transceivers in a wired or wirelessmulticarrier modulation communications network to regulate the data ratebetween the transceivers.

These and other features and advantages of this invention are describedin, or apparent from, the following detailed description of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be described in detailed, withreference to the following figures, wherein:

FIG. 1 is a functional block diagram illustrating an exemplarycommunication system according to this invention;

FIG. 2 is a functional block diagram illustrating the components of afirst and a second transceiver according to this invention;

FIG. 3 is a flowchart illustrating an exemplary communication methodaccording to this invention;

FIG. 4 illustrates an exemplary extended signal field according to thisinvention;

FIG. 5 illustrates a second exemplary communication system according tothis invention; and

FIG. 6 illustrates an exemplary transceiver in accordance with thesecond exemplary embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The exemplary systems and the methods of this invention will bedescribed in relation to a multicarrier modulation communication system.However, to avoid unnecessarily obscuring the present invention, thefollowing description omits well-known structures and devices that maybe shown in block diagram form or otherwise summarized. For the purposesof explanation, numerous specific details are set forth in order toprovide a thorough understanding of the present invention. It should beappreciated however that the present invention may be practiced in avariety of ways beyond the specific details set forth herein. Forexample, the systems and methods of this invention can generally beapplied to any type of communications system including wiredcommunication systems, wireless communication systems, such as wirelessLANs, power line communication systems, wired or wireless telephone linecommunication systems, or any combination thereof.

Furthermore, while the exemplary embodiments illustrated herein show thevarious components of the communication system collocated, it is to beappreciated that the various components of the system can be located atdistant portions of a distributed network, such as a telecommunicationsnetwork and/or the Internet, or within a dedicated multicarriermodulation system. Thus, it should be appreciated that the components ofthe communication system can be combined into one or more devices orcollocated on a particular node of a distributed network, such as atelecommunications network. It will be appreciated from the followingdescription, and for reasons of computational efficiency, that thecomponents of the communications system can be arranged at any locationwithin a distributed network without affecting the operation of thesystem.

Furthermore, it should be appreciated that the various links connectingthe elements can be wired or wireless links, or any combination thereof,or any other known or later developed element(s) that is capable ofsupplying and/or communicating information to and from the connectedelements. Additionally, the term module as used herein can refer to anyknown or later developed hardware, software, or combination of hardwareand software that is capable of performing the functionality associatedwith that element.

Additionally, while this invention will be described in the relation tomulticarrier modulation systems, the systems and methods of thisinvention can be applied to any communication system or transportprotocol for transmitting information.

FIG. 1 illustrates an exemplary communication system 1. Communicationsystem 1 comprises one or more stations 10 and an access point (AP) 20.This exemplary embodiment illustrates a wireless LAN where a pluralityof stations 10 communication with the access point 20. In particular, inits exemplary wireless LAN, multiple stations 10 share a commoncommunication medium. One possible configuration includes an accesspoint 20 that is used to communicate between the stations 10 (BSS). Theaccess point 20 provides the local relay functionality between thestations 10 and to, for example, other wired and/or wireless networks(not shown). Therefore, when station 1 communicates with station 2, thecommunication, e.g., a packet, is sent from station 1 to the accesspoint 20, and then from the access point 20 to station 2. For thisreason, in most cases a station 10 is only transmitting packets to theaccess point 20 and receiving packets from the access point 20. Theaccess point 20 on the other hand, must communicate with all thestations 10 in the network.

Another possible configuration does not rely on an access point 20, butinstead communications take place directly between the stations 10(IBSS) in the network illustrated by the dashed lines in FIG. 1. In thisembodiment, where communications occur directly between the stations 10,there are no relay functions served by the access point 20.

In accordance with an exemplary embodiment of this invention, thewireless network relies on communicating parameters between a pluralityof transceivers and in particular from a receiver to a transmitter.These parameters are stored at the transmitter and are used forsubsequent transmission of packets to the receiver the parameters werereceived from. Thus, the systems and methods of this invention will workequally well whether the network is configured to have an access point20, or not, as each station, including the access point, if used,maintains tables comprising the communication parameters.

Several different types of communication parameters can be sent from thereceiver to the transmitter to optimize communication to, for example,increase or decrease the data rate. In general, any parameter that canmodify performance can be included in the message. The followingexamples are the more common types of communication parameters that canbe exchanged between the receiver and the transmitter.

The Bit Allocation Table (BAT)—the bit allocation table in multicarriermodulation systems specify the number of bits modulated on each carrier,which are also referred to as subchannels, subcarriers, tones or bins,in a multicarrier modulation system. The 802.11a/g transceivers use thesame number of bits on all subchannels, which is the simplest type ofbit allocation table. Since wireless communications experiencemultipath, the communications channel is not flat in frequency, whichmeans that different subcarriers will have different signal to noiseratios (SNRs). Therefore, in order to achieve a constant bit error rate(BER) on all carriers, a bit allocation table is used so that carrierswith a higher SNR modulate more bits than carriers with a lower SNR.This process is often referred to as “bit loading.” Bit loading and theuse of a bit allocation table has been used in ADSL multicarriercommunication systems for years. For example, ITU standards G.992.1 andG.992.2, which are incorporated herein by reference in their entirety,are international ADSL standards that specify communication using bitloading and bit allocation tables. Bit loading also enables usingconstellation sizes much higher than 64 QAM (6 bit) which is the maximumconstellation size of standard 802.11a/g systems. Bit loadingconstellations that modulate up to 15 bits, or more, can be used, ifsupported by the channel, thereby achieving significant data rateimprovements.

Coded modulation parameters—systems that use coded modulationtechniques, such as trellis coded modulation and turbo coded modulation,achieve much higher coding advantages than systems that do not combinemodulation and forward error correction encoding. However, codedmodulation schemes do not encode all information bits and thereforecoded modulation must be combined with bit loading in multipath channelsin order to achieve the coding gain benefits.

Variable cyclic prefix length—the cyclic prefix (CP) is used inmulticarrier systems to combat multipath. In general, as long as theimpulse response of the channel is less than the CP length, there willbe no inter-symbol interference (ISI) or inter-channel (ICI)interference due to the channel multipath. However, since the CP is aredundant cyclic extension added to every communication symbol, the CPalso results in a data rate loss. The 802.11a/g standards use a fixed CPwith a length of 0.8 microseconds, which is 20% of the symbol length.Therefore, the addition of the CP results in a 20% data rate reduction.This is a good tradeoff if the channel is approximately the same lengthas a CP. However, if the channel is much shorter, e.g., only 0.1microseconds, then it makes sense to decrease the CP length to 0.1microseconds in order to get a 19% data rate improvement. Likewise, ifthe channel is much longer than 0.8 microseconds, the CP should beextended to match the length of the channel because significant levelsof ISI and ICI will probably greatly reduce the achievable data rate.

Variable pilot tone allocation—standard 802.11a/g receivers use fourfixed pilot tones that are spread across the transmission frequencyband. This is necessary in 802.11a/g systems since the transmitter doesnot know which portions of the frequency bands are in deep nulls due tomultipath. In accordance with an exemplary embodiment of this invention,the receiver can communicate to the transmitter which carrier should beused for pilot tones. Since the receiver can determine which carriershave a high SNR, the receiver can instruct the transmitter to placepilot tones on those high SNR carriers. In fact, in many cases, a singlehigh SNR carrier is sufficient to be used for all timing recoveryrequirements thereby allowing the system to transmit data on the threecarriers that the 802.11a/g systems use for pilot tones. This alsoprovides a data rate increase when compared to standard 802.11a/gsystems.

Alternatively, the communication system may not have any carriersdedicated as pilot tones, i.e., all carriers that are modulated aremodulated with information bits. In this case, a carrier that carriesinformation bits may be used to perform “decision-directed” timingrecovery algorithms. For example, a carrier that is used for this typeof decision-directed algorithm will often carry fewer bits than actuallypossible at the specified BER in order to provide a reference signalwith a high SNR.

Fine gains per carrier—Fine gains are used in ADSL standards such asG.992.1 to equalize the BER across all the carriers when bit loading isused. Fine gains are small adjustments in the transmit power level thatenable a subchannel to achieve the BER required by the system based onthe specific measure of SNR.

Throughout the following discussion, exemplary embodiments of thisinvention will be directed toward the bit allocation tables (BATs) asthe primary optimized communication parameter that is being exchangedbetween the stations. This is done because the use of BATs is one of themost effective ways to achieve optimized communication and to modifydata rates. However, it is to be appreciated that other communicationparameters including, but not limited to, fine gains, trellis codedmodulation, pilot tone location, variable cyclic prefix length, and thelike, can also be exchanged, with or without BATs, between stations torealize a change in data rate.

To implement a change in data rates, a message containing thecommunication parameters is sent from a receiver to a transmitter. Thesecommunication parameters can be communicated in a plurality of ways. Forexample, the communication parameters can be sent to the transmitter aspart of a positive acknowledgment packet. In this case, after receivingthe positive acknowledgment packet, the transmitter would use thecommunication parameters contained in the positive acknowledgment packetfor the transmission of subsequent packets. The communication parameterscould also be sent, for example, as part of a management or data framethat is intended to communicate information between the transceivers.For example, the communication parameters could be sent as part of anextended header field of any packet sent between the transceivers.

The exemplary embodiment of the protocol used for exchangingcommunication parameters in accordance with an exemplary embodiment ofthis invention will be discussed in relation to FIGS. 1 and 2. Inparticular, FIG. 1 illustrates an exemplary network 1, such as awireless network. The network 1 comprises a plurality of stations 10interconnected by a plurality of links and an access point 20. FIG. 2illustrates an exemplary embodiment of the components associated with afirst and a second transceiver, e.g., the stations 10 or the accesspoint 20. In particular, the first transceiver 100 comprises a messagedetermination module 110, a communication parameter determination module120, a packet determination module 130, a transmitter 140, a receiver150, a memory 160, and a controller 170, all connected by a link (notshown). The second transceiver 200 comprises a message determinationmodule 210, a communication parameter determination module 220, a packetdetermination module 230, a transmitter 240, a receiver 250, a memory260, and a controller 270, all connected by a link (not shown).

For ease of illustration the exemplary method used for the high rateOFDM communication systems will be discussed in relation to a firsttransceiver sending packets to a second transceiver. For example, thefirst transceiver could be station 2 and the second transceiver theaccess point 20. Alternatively, the first transceiver could be station 2and the second transceiver, station 1, or the like. The relevant portionof the protocol commences with the first transceiver sending a packet atone of a highest possible data rate, e.g., 54 Mbps for 802.11a/g, at thedata rate of the last successful transmission, or at a known data rate.

Specifically, the packet determination module 130, in cooperation withthe transmitter 140, the memory 160 and the controller 170 coordinatethe transmission of this first packet, i.e., before any optimizedcommunication parameters are exchanged, and transmit the packet usingstandard size fixed communication parameter settings such as thosespecified in IEEE 802.11a/g, e.g., fixed six bits per tone on allcarriers.

Next, if the second transceiver's receiver 250 successfully receives thepacket from the first transceiver 100, the second transceiver 200returns to the first transceiver a positive acknowledgment packet againwith the cooperation of the packet determination modulation 230, thetransmitter 240, the memory 260 and the controller 270. This positiveacknowledgment packet also comprises optimized communication parametersdetermined by the communication parameter determination module 220 to beused by the second transceiver 200 for subsequent reception of packetsfrom the first transceiver 100. For example, the positive acknowledgmentpacket may contain a BAT with different bits per subcarrier based on,for example, the channel characteristics as measured by the secondtransceiver 200 and determined by the communication parameterdetermination module 220. Alternatively, or in addition, thisacknowledgment packet may also indicate any of the optimizedtransmission parameters described above, e.g., which one or morecarriers should be used as pilot tones as discussed above.

If the second transceiver 200 does not successfully receive the packetfrom the first transceiver 100, the second transceiver 200 does notreturn to the first transceiver a positive acknowledgment packet. Inthis case, the first transceiver 100, again in cooperation with thepacket determination module 130, the transmitter 140, the memory 160 andthe controller 170, sends a packet at the next highest or another knownstandard data rate.

If the first transceiver 100 receives the positive acknowledgmentpacket, the first transceiver 100, in cooperation with memory 160 storesthe optimized communication parameters. The first transceiver 100 thenuses the stored communication parameters for transmission of subsequentpackets to the second transceiver 200. The use of the optimizedcommunication parameters is indicated in the header field of the packetsent from the first transceiver 100 to the second transceiver 200. Forexample, the message determination module 110 modifies the header fieldto indicate which optimized communication parameters are being used.

The second transceiver's receiver 250 receives the packet from the firsttransceiver 100 and determines which communication parameters were usedbased on the information in the data field of the packet. This isaccomplished by, for example, decoding the header field of the packetthat indicates that optimized communication parameters are being used.The packet can then be demodulated and decoded based on the informationcontained in the data field in association with the messagedetermination/decoded module 210 using the optimized communicationparameters that were sent from the second transceiver to the firsttransceiver in the previous positive acknowledgement packet.

After the second transceiver 200 receives from the first transceiver 100the packet which has the header field specifying which optimizecommunication parameters were used, the second transceiver 200 sends apositive acknowledgment back to the first transceiver 100. This positiveacknowledgment may contain the same parameters as used for the lastsuccessful received packet as an indication to the second transceiver200 to continue transmitting with the stored optimized communicationparameters. Equivalently, the positive acknowledgment may be just abasic acknowledgment packet, as in conventional 802.11a/g systems, toindicate that the packet was successfully received at the secondtransceiver and communication should continue using the same optimizedcommunication parameters. In the event that optimized communicationparameters accompany every positive acknowledgement during an extendedcommunication session, this mechanism effectively tracks

Alternatively, the second transceiver 200 may send a new, second set ofoptimized communication parameters in the acknowledgment message. Thesenew parameters could, for example, request a change in data rate, suchas a higher data rate. In this case, the first transceiver 100 couldstart using the second set of optimized communication parameters fortransmission after receiving the acknowledgment packet.

In the case where the second transceiver 200 does not successfullyreceive the packet transmitted by the first transceiver 100 that has themodified header field specifying which communication parameters wereused, the second transceiver 200 will not send a positive acknowledgmentback to the first transceiver 100. In this case, the first transceiver100 would determine that the optimized communication parameters are nolonger valid and will start the protocol all over again by going back tothe first step were the first transceiver 100 will commencecommunication at a known data rate, such as the highest data rate, e.g.,54 Mbps in 802.11a/g systems, using the fixed/standard communicationparameters.

In the case were the first transceiver 100 receives the positiveacknowledgment from the second transceiver 200 after transmitting apacket using the first set of optimized parameters, and this positiveacknowledgment contains a new, second set of optimized parameters, thesenew parameters should be used for subsequent transmission of packets.However, if the second transceiver 200 does not receive a positiveacknowledgment packet after sending a packet using the second set ofoptimize parameters, then the second transceiver 200 reverts back to thefirst step of the protocol were a packet is sent at a known e.g., nexthighest data. However, in this case, the first transceiver may start bytransmitting using the first set of optimized communication parametersor by transmitting at a data rate using a fixed/standard communicationparameter, e.g., 54 Mbps in the 802.11a/g standard.

Alternatively, or further in addition, the first transceiver 100 and thesecond transceiver 200 may periodically send “reference” or “training”packets that can be used by the receiver portion of the transceiver inconjunction with the communication parameter determination module todetermine the optimized transmission parameters. For example, thesetraining packets can be packets that contain signals that are known tothe transceivers in advance. For example, the training packets can benon-information carrying packets that are sent during times when thereis no data to be sent between the stations and the network. Since thesepackets are predefined and known to the receiver prior to reception, thereceivers can use them to accurately measure the effects of the channel,such as the multipath profile, the SNR per carrier, or the like. Thesetraining packets can also be used to train receiver equalizers that areused to equalize, for example, the wireless channel and/or receiverfilters and/or transmitter filters.

In conventional wireless LAN systems, every packet contains a headerfield that indicates the data rate used for transmitting the data fieldof the packet. The header field is transmitted using a fixedmodulation/encoding scheme, such as in the 802.11a/g standard, andtherefore can be demodulated by all stations. In accordance with anexemplary embodiment of this invention, the header field will alsoindicate whether optimized communication parameters were used fortransmitting the data field in the packet. This could be done in severalways. For example, the header field could contain the indication of thedata rate as in 802.11a/g. Alternatively, the header field could containa bit field that indicates whether the optimized communicationparameters are to be used. This bit field could be a single bit thatindicates either to use the last exchanged optimized communicationparameters, or one of the standard fixed communication parameters.Alternatively, the bit field could be a plurality of bits indicating oneof a plurality of sets of optimized communication parameters.

In the example of a network with a access point 20, each stationtransmitter would store optimized communication parameters to be usedwhen sending packets to the access point 20. These optimized parameterswould be generated by the access point 20 receiver and sent to thestation(s) as described above. Obviously, since each station 10 is in adifferent location, and could possibly move, each station transmitterwould probably have different optimized parameters to be used whensending packets to the access point 20. The access point 20 must alsostore these optimized parameters to be used by the access point 20receiver when receiving packets from the various stations 10. For eachstation 10, the access point 20 may have a different set of optimizeparameters. Since the access point 20 receives packets from allstations, the access point 20 must be able to determine the parametersused for the data field based on the information in the packet header,i.e., the SIGNAL field. The access point 20 can use the packet header todetermine whether the optimized parameters have been used, but since theaccess point 20 does not know which station actually sent the packet,the access point 20 may not be able to determine the correct parametersbased on the header alone.

Accordingly, and in accordance with the exemplary embodiment of thisinvention, the header also includes a bit field that indicates whichstation sent the packet. In this case, the access point 20 would usethat information to determine which set of parameters should be used.Alternatively, the access point 20 may use other measures to determinewhich station sent the packet. For example, the access point 20 coulduse the power of the received signal, the channel estimate based onfrequency equalizer taps, carrier offset values, or the like.

In the example of a network, such as a wireless LAN, with an accesspoint 20, the access point transmitter would store the optimizedcommunication parameters to be used when sending packets to a specificstation 10. These optimized communication parameters would be generatedby the station receiver and sent to the access point 20 as describedabove. Obviously, since each station is in a different location, theaccess point 20 could have a plurality of sets of different optimizedcommunication parameters to be used when sending packets to thedifferent stations 10. Each station 10 would then also store theoptimize communication parameters corresponding to that station to beused by the station receiver when receiving packets from the accesspoint 20. Each station 10 should also be able to determine thecommunication parameters used for the data field based on theinformation in the packet header, i.e., SIGNAL field. Therefore, eachstation 10 uses the packet header to determine whether the optimizedcommunication parameters have been used. Unlike the access pointreceiver, each station receiver is intended to receive packets only fromthe access point 20 and therefore a station 10 may be able to determinethe communication parameters based on the header alone.

Since all stations will receive packets from the access point 20, eachstation must also be able to determine the communication parameters usedfor the data field based on the preamble and the packet header, i.e.,the SIGNAL field. Obviously, if the packet is not intended for aparticular station receiver, the receiver may use the incorrectoptimized communication parameters to receive a packet. This is actuallynot a problem since the packet was not intended for that receiver in thefirst place. However, since the protocol requires transmitters to deferto communications already in progress, every station must be able todetermine various protocol counters based on the packet duration. Theheader must provide a way to determine the packet duration even if useof the wrong communication parameters does not permit the receiver tocorrectly decode the message.

As discussed above, once the receiving transceiver determines theoptimized transmission parameters, the receiving transceiver needs tosend this information to the transmitting transceiver to be used forsubsequent communication between the two devices. Furthermore, asdiscuss above, the optimized transmission information can be sent aspart of an acknowledgment packet. Alternatively, or in addition, theoptimized transmission parameters can be exchanged as part of amanagement frame or regular information carrying frame on a periodic or,for example, triggered basis. In either case, the optimized transmissionparameters can be sent as part of an extended packet header field, alsoknown as the SIGNAL field, or as part of the packet information field.In the case of an extended packet header field, the information is sentat a fixed rate and can be decoded by all systems in the network. Forexample, a bit in the packet header field can be used to indicate that anew set of optimized transmission parameters has been appended to anextended packet header field.

In the latter case, the information can be sent using optimizedparameters for communication. Note that in this case the optimizedtransmission parameters that are used for transmitting the optimizedtransmission parameter information from the receiver to the transmitterare not the same. For example, assume that the receiver of the firsttransceiver 150 determines optimized transmission information fortransmitting packets from the second transceiver's transmitter 240 tothe first transceiver's receiver 150. The first transceiver'stransmitter 140 sends a packet to the second transceiver's receiver 250where the packet contains the optimized transmission parameters fortransmitting packets from the second transceiver's transmitter 240 tothe first transceiver's receiver 150. The packet that is sent from thefirst transceiver's transmitter 140 to the second transceiver's receiver250 may be sent using a standard fixed rate, as is done in conventional802.11a/g systems, or may be sent using optimized transmissionparameters communicated between the first transceiver 100 and the secondtransceiver 200. Obviously, the optimized transmission parameters usedfor transmission from the first transceiver 100 and the secondtransceiver 200 would have been exchanged earlier in the communicationssession.

FIG. 3 is a flowchart illustrating a general exemplary method ofexchanging communication parameters according to this invention.Specifically, control begins in step S100 and continues to step S110. Instep S110, a first transceiver (designated T1) determines and sends apacket that is at least one of a known, highest, last successful orchanged rate to a second transceiver (designated T2). Next, in stepS120, a determination is made whether the packet was successfullyreceived at the second transceiver. If the packet was not successfullyreceived, control jumps to step S130. Otherwise, control continues tostep S140.

In step S130, the communication parameters specifying the data rate areincremented/decremented as appropriate. Control then continues back tostep S110.

In step S140, the second transceiver returns to the first transceiver apositive acknowledgment that may or may not comprise optimizedcommunication parameters. If the positive acknowledgement containsoptimized communication parameters, the second transceiver stores theseparameters. Next, in step S150, the first transceiver receives theacknowledgment. Then, in step S160, the first transceiver stores theoptimized communication parameters if the positive acknowledgmentreturned from the second transceiver contains communication parameters.Control then continues to step S170.

In step S170, the first transceiver determines a header field. Next, instep S180, the first transceiver commences communication using thestored optimized communication parameters. Then, in step S190, adetermination is made whether the second transceiver received thepacket. If the packet was received, control continues to step S200.Otherwise, control jumps to step S130.

In step S200, the second transceiver decodes the header field anddetermines the communication parameters that were used. Next, in stepS210, the second transceiver demodulates and decodes the data fieldusing the stored optimized communication parameters. Then, in step S220,the second transceiver determines the acknowledgment to return to thefirst transceiver. Control then continues to step S230.

In step S230, the second transceiver sends the acknowledgment to thefirst transceiver. This message may or may not contain optimizedcommunication parameters. Control then continues to step S240 where thecontrol sequence ends.

The basic concepts discussed above can also be extended to legacysystems. In the following discussion, stations that only implement thecurrent 802.11a/g standard will be referred to as legacy stations.Stations that are enabled with the methods of this invention to providehigh data rate communications with optimized communication parameterswill be referred to as extended rate (ER) stations. The method andprotocols that enable exchanging, transmitting and receiving using theseoptimized communication parameters are referred to as extended ratesystems and protocols. In this exemplary embodiment, an extended ratestation also supports the current 802.11a/g standard.

For example, FIG. 5 illustrates an exemplary communication system 500that comprises a plurality of extended rate stations 510, 520, one ormore legacy stations 530 and, for example, an access point 540.

When operating in an environment with legacy stations 530 and extendedrate stations 510, 520 there are two main interoperability requirementsto ensure network stability. First, a legacy station 530 must be able toreceive the ER packet header (SIGNAL field) and use the SIGNAL fieldparameters to correctly determine the packet duration, i.e., the timerequired for packet transmission. This will guarantee that the legacystation 530 will correctly set its network allocation vector (NAV) andother related counters so that accurate operation of the contentionalgorithm for the medium access will be maintained.

Secondly, an extended rate station 510, 520 must be able to determinethe transmission parameters e.g., the bit allocation table, based on anextended rate packet header if the packet is intended for that station.In addition, an extended rate station that was not intended to receivethe packet must also use the SIGNAL field parameters to correctlydetermine the packet duration, i.e., the time required for packettransmission. This will ensure that the extended rate station willcorrectly set its network allocation vector (NAV) and other relatedcounters so that accurate operation of the contention algorithm for themedium access will be maintained.

In an effort to ensure the two above requirements are met, FIG. 4illustrates an exemplary modified packet header using an extended signalfield. In this illustrative 802.11a example, the SIGNAL field isextended. The first part of the extended SIGNAL field has a structureidentical to the standard 802.11a SIGNAL field header. The first symbolof the extended SIGNAL field is modulated according to the SIGNALmodulation encoding parameters as specified in IEEE 802.11a for thestandard SIGNAL field, i.e., 6 Mbps BPSK, code rate=½. Therefore, alegacy station can correctly receive the signal field bits from thefirst part of the extended SIGNAL field.

The second part of the extended signal field in the next symbol containsthe transmitter (TX) and receiver (RX) station identifiers. Theseextended signal field bits are also modulated using the 802.11a 6 Mbps,code rate=½ modulation method. In FIG. 4, these extended signal fieldbits are sent in the second symbol of the extended signal field headerthat corresponds to the data symbol number one in a standard 802.11asystem.

Since there are both legacy and extended rate stations in the exemplarycommunication system 500 illustrated in FIG. 5, an extended rate stationneeds to be able to determine and identify when a received packetcontains an extended signal field header, which is contained in twosymbols, as opposed to a standard 802.11a header, which contained inonly one symbol. This can be accomplished by setting a bit in thestandard 802.11a SIGNAL field. This bit will be referred to as theER-enable bit. As an example, the 802.11a reserved bit between the ratefield and the length field can be used as the ER-enable bit. Forexample, when this reserved bit (R) is set to 1, this indicates that anextended rate header is being used. When the reserved bit (R) is set to0, this indicates that a standard 802.11a header is being used.

Again with reference to FIG. 5, two ER stations 510 and 520 areillustrated along with a legacy station 530 and an extended rate accesspoint 540. The various links in FIG. 5 represent, for example, thecommunication paths of an extended rate packet where the ER-enable bit(R) is flagged in the reserved bit R position and the TX/RX STA ID(Transmitter/Receiver Station Identifier) is present in the extendedSIGNAL field.

The exemplary communications that occur between the various stationswill be discussed in relation to FIGS. 5 and 6. In particular, FIG. 6illustrates the exemplary components that could be present in a stationillustrated in FIG. 5. In particular, the station 600 comprises amessage determination module 610, a communication parameterdetermination module 620, a packet determination module 630, an ERdetection module 640, a station ID decoder/encoder 650, a receiver 660,a transmitter 670, a memory 680 and a controller 690. Many of thecomponents illustrated in the station 600 are comparable to those seenin the first transceiver 100 and second transceiver 200. Accordingly,the functions of those components will not be re-discussed inassociation with this embodiment of the invention.

Communication path 1: Transmission of packets from the access point 540to a ER capable station, such as ER station 510.

The access point 540 forwards to the ER station 510 a packet. The ERstation 510 detects the ER-enable bit with the cooperation of the ERdetection module 640 and determines that the packet is an ER packet.

Next, the station ID decoder/encoder 650 decodes the RX STA ID bits andthe extended header field to determine if the received packet isintended for this particular station. The ER station 510 also decodesthe TX STA ID in the extended rate header with the cooperation of thestation ID decoder/encoder 650 and determines if this packet is comingfrom the access point 540. Based on this information, the receivingextended rate station 510 uses the stored optimized communicationparameters that are to be used when receiving the packets from theaccess point 540. The extended rate station 510 uses the optimizeparameters to correctly decode the remainder, i.e., the data field, ofthe packet. Naturally, the RX station had sent these optimizedcommunication parameters to the AP earlier in the session.

Communication Path 2: Another ER-capable station, e.g., station 520,accidentally receives a packet from the access point (AP) 540.

The station 520, in cooperation with the ER detection module 640,detects the ER-enable bit in the packet sent from the access point 540,and determines that the packet is an ER packet and, with the cooperationof the STA ID de/encoder 650, decodes the RX STA ID bits in the extendedheader field and determines that the received packet is not intended forthis particular station. The station 520 then sets the NAV, and relatedcounters, based on the “spoofed” RATE, LENGTH information contained inthe SIGNAL Field, as discussed below.

Since the station 520 determines that the received packet is notintended for itself, the station 520 does not even have to decode thepacket. An additional benefit of this method is that when a packet isreceived, a station can detect very early whether it is the intendedrecipient of the packet and therefore the station does not need todecode the remainder of the packet if it is not. This will, for example,save power in the station since the station will not consume theprocessing power required to decode the remainder of the packet andtherefore, for example, the station may go into a low power mode.

Communication Path 3: The legacy station 530 accidentally receives apacket originating from the access point (AP) 540.

Legacy stations in general are not aware of ER packet headers.Therefore, the legacy station 530 will correctly decode the first partof the ER packet which is contained in the first symbol of the headerfield and is identical to the standard 802.11a SIGNAL field, except forthe ER-enable bit which the legacy station 530 should ignore since it isreserved.

The legacy station 530 sets the NAV, and related counters, based on the“spoofed” RATE/LENGTH information contained in the SIGNAL Field asdiscussed below allowing correct legacy operation of the 802.11a mediumoccupancy algorithms. Using the spoofed RATE and LENGTH information, thelegacy station 530 will incorrectly demodulate the data symbols, sincethe station does not know the optimized communication parameters, untileventually a CRC error will cause the packet to be ignored.

Communication Path 4: Transmission of packets from an ER-capable station510 to an access point (AP) 540.

The access point (AP) 540 detects the ER-enable bit, determines thereceived packet is an ER packet and decodes the RX STA ID bits in theextended header field to determine if the packet is intended for itself.The access point 540 also decodes the TX STA ID in the ER header anddetermines which station has transmitted the packet. Based on thisinformation, the access point 540 uses the stored optimizedcommunication parameters that are to be used when receiving packets fromthat particular transmitter station. The access point 540 then uses theoptimized parameters to correctly decode the remainder, i.e., DATAField, of the packet. Of course, the access point 540 had sent theoptimized communication parameters to the transmitter station earlier inthe communications session.

Communication Path 5: Another ER-capable station 520 accidentallyreceives a packet originating from ER station 510.

The station 510 detects the ER-enable bit, with the cooperation of theER detection module, determines this is an ER packet and decodes, withthe cooperation of the STA ID de/encoder 650, the RX ST ID bits in theextended header field to determine that the packet is not intended foritself The station 510 then sets the NAV, and related counters, based onthe “spoofed” RATE, LENGTH information contained in the SIGNAL Field asdiscussed below. Since the station 510 knows that this packet is notintended for itself, the station 510 does not even have to decode thepacket. An additional benefit of this method is that when this happens,a station can detect very early that it is not the intended recipient ofthe packet and therefore the station does not need to decode theremainder of the packet. This will save, for example, power in thestation since the station will not consume the processing power todecode the remainder of the packet and therefore the station may, forexample, go into the low power mode.

Communication Path 6: Legacy station 530 accidentally receives a packetoriginating from an ER-enabled station 510.

This scenario produces the same results as illustrated in relation tocommunication path 3.

“Spoofing” the RATE and LENGTH Field.

When a legacy station receives an ER packet, such as in communicationpaths 3 and 6, the legacy station must be able to determine the durationof the packet, i.e., the time required for packet transmission, based onthe standard 802.11a header contained in the first symbol of the ERpacket header, which every station can correctly decode. Thus, for thelegacy station, R1-R4 bits, which do not have any meaning to theER-capable RX STA, must be set to one of the legitimate patterns used inthe 802.11a standard, shown in Table 1. Additionally, the LENGTH fieldmust be filled in conjunction with the RATE field in a way that therequired time for packet transmission that the legacy RX STA wouldcalculate based on the “spoofed” RATE and LENGTH parameters wouldcoincide with the one that is needed by the ER RX STA using optimizedcommunication parameters. This will guarantee that the legacy stationwill correctly set its network allocation vector (NAV) and other relatedcounters so the accurate operation of the contention algorithm for themedium access will be maintained.

A ER-capable RX STA will also exploit the spoofed RATE, LENGTHinformation shown in the SIGNAL field when the packet is not intendedfor its reception, such as in cases 2 and 5. Once the ER-capable RX STArecognizes that the reserved bit R is turned on, the ER-capable RX STAexamines the extended SIGNAL symbol and, based on the RX STA ID,determines that this packet is not intended for itself. Based on the‘spoofed’ RATE and LENGTH information in the SIGNAL Field, the RX STAsets the counters related with virtual carrier sense algorithm inexactly the same manner as the legacy station and may then enter thepower saving mode.

As an example, the ER data rate is 108 Mbps, which is twice the maximumdata rate (54 Mbps) of conventional 802.11a systems. This may beachieved by, for example, bit loading and using trellis codedmodulation. A system transmitting at 108 Mbps will have 432 data bitsper symbol. Therefore, transmitting a packet with, for example, 864bytes will require 864*8/432=16 symbols. In addition, the ER protocolrequires an extra symbol in the ER header, as compared to standard802.11a systems, that contains the TX and RX Station IDs. Therefore, thetransmission of an 864 byte packet requires 16+1=17 symbols at 108 Mbps.In order to allow legacy 802.11a stations to correctly determine theNAV, the RATE and LENGTH Fields of the ER header need to be set so thatthe legacy station will also determine that 17 symbols are needed fortransmission of the packet. Therefore, for example, the RATE and LENGTHfields could be set to RATE=54 Mbps and LENGTH=459 bytes. In this case,since 54 Mbps results in 216 data bits per symbol, the legacy stationwould determine the packet duration to be 459*8/216=17 symbols andcorrectly set the NAV. Obviously other RATE and LENGTH combinations canbe used from the 802.11a standard to enable the legacy station tocorrectly set the NAV. For example, RATE=6 Mbps and LENGTH=51 byteswould also result in a packet whose data field is 17 symbols long.

In the example described above, the extended header field only containedthe RX and TX STA IDs. This implies that there is only one set ofoptimized parameters for each TX/RX communication. In an alternativeembodiment, the extended header field also, or alternatively, containsan indication of which one of a plurality of optimized communicationparameters sets is to be used for transmission and reception of apacket. These parameter sets are sent from the receiver station to thetransmitter station and stored in each.

The above-described communication system can be implemented on wired orwireless telecommunications devices, such a modem, a multicarrier modem,a DSL modem, an ADSL modem, an XDSL modem, a VDSL modem, a multicarriertransceiver, wired or wireless wide/local area network system, or thelike, or on a separate programmed general purpose computer having acommunications device. Additionally, the systems, methods and protocolsof this invention can be implemented on a special purpose computer, aprogrammed microprocessor or microcontroller and peripheral integratedcircuit element(s), an ASIC or other integrated circuit, a digitalsignal processor, a hard-wired electronic or logic circuit such asdiscrete element circuit, a programmable logic device such as PLD, PLA,FPGA, PAL, modem, transmitter/receiver, or the like. In general, anydevice capable of implementing a state machine that is in turn capableof implementing the flowcharts illustrated herein can be used toimplement the various communication methods according to this invention.

Furthermore, the disclosed methods may be readily implemented insoftware using object or object-oriented software developmentenvironments that provide portable source code that can be used on avariety of computer or workstation platforms. Alternatively, thedisclosed communication system may be implemented partially or fully inhardware using standard logic circuits or VLSI design. Whether softwareor hardware is used to implement the systems in accordance with thisinvention is dependent on the speed and/or efficiency requirements ofthe system, the particular function, and the particular software orhardware systems or microprocessor or microcomputer systems beingutilized. The communication systems, methods and protocols illustratedherein however can be readily implemented in hardware and/or softwareusing any known or later developed systems or structures, devices and/orsoftware by those of ordinary skill in the applicable art from thefunctional description provided herein and with a general basicknowledge of the computer and telecommunications arts.

Moreover, the disclosed methods may be readily implemented in softwareexecuted on programmed general purpose computer, a special purposecomputer, a microprocessor, or the like. In these instances, the systemsand methods of this invention can be implemented as program embedded onpersonal computer such as JAVA® or CGI script, as a resource residing ona server or graphics workstation, as a routine embedded in a dedicatedcommunication system, or the like. The communication system can also beimplemented by physically incorporating the system and method into asoftware and/or hardware system, such as the hardware and softwaresystems of a communications transceiver.

It is therefore apparent that there has been provided, in accordancewith the present invention, systems and methods for exchangingcommunication parameters. While this invention has been described inconjunction with a number of embodiments, it is evident that manyalternatives, modifications and variations would be or are apparent tothose of ordinary skill in the applicable arts. Accordingly, it isintended to embrace all such alternatives, modifications, equivalentsand variations that are within the spirit and scope of this invention.

1. A communications system comprising an OFDM transceiver capable of:transmitting or receiving, over a first communication channel using afirst cyclic prefix length, a first plurality of OFDM symbols that carrydata bytes; detecting a change in one or more channel conditions; andtransmitting or receiving, over a second communication channel using asecond cyclic prefix length, a second plurality of OFDM symbols thatcarry data bytes; wherein: the first cyclic prefix length is shorterthan the second cyclic prefix length, the first communication channelhas a shorter impulse response length than the second communicationchannel, the first cyclic prefix length is based on a length of animpulse response for the first communication channel, and the secondcyclic prefix length is based on a length of an impulse response for thesecond communication channel.
 2. The system of claim 1, wherein amessage or a management frame indicates the length of the cyclic prefix.3. The system of claim 1, wherein a SIGNAL field indicates the length ofthe cyclic prefix.
 4. The system of claim 1, wherein the transceiverselects the length of the cyclic prefix.
 5. The system of claim 1,wherein a second transceiver selects the length of the cyclic prefix. 6.The system of claim 1, wherein the transceiver is a wireless 802.11a ora 802.11g transceiver.
 7. In an OFDM transceiver, a method comprising:transmitting or receiving, over a first communication channel using afirst cyclic prefix length, a first plurality of OFDM symbols that carrydata bytes; detecting a change in one or more channel conditions; andtransmitting or receiving, over a second communication channel using asecond cyclic prefix length, a second plurality of OFDM symbols thatcarry data bytes, wherein: the first cyclic prefix length is shorterthan the second cyclic prefix length, the first communication channelhas a shorter impulse response length than the second communicationchannel, the first cyclic prefix length is based on the length of animpulse response for the first communication channel, and the secondcyclic prefix length is based on the length of the impulse response forthe second communication channel.
 8. The method of claim 7, wherein amessage or a management frame indicates the length of the cyclic prefix.9. The method of claim 7, wherein a SIGNAL field indicates the length ofthe cyclic prefix.
 10. The method of claim 7, further comprisingselecting, by the transceiver, the length of the cyclic prefix.
 11. Themethod of claim 7, further comprising selecting, by a secondtransceiver, the length of the cyclic prefix.
 12. The method of claim 7,wherein the transceiver is a wireless 802.11 a or a 802.11g transceiver.13. A communications system comprising an OFDM transceiver capable of:transmitting or receiving, over a first communication channel using afirst cyclic prefix length, a first plurality of OFDM symbols that carrydata bytes; and changing from a first data rate to a second data rate bytransmitting or receiving, over a second communication channel using asecond cyclic prefix length, a second plurality of OFDM symbols thatcarry data bytes, wherein: the first cyclic prefix length is shorterthan the second cyclic prefix length, the first communication channelhas a shorter impulse response length than the second communicationchannel, the first cyclic prefix length is based on the length ofimpulse response of the first communication channel, and the secondcyclic prefix length is based on the length of the impulse response ofthe second communication channel.
 14. The system of claim 13, wherein amessage or a management frame indicates the length of the cyclic prefix.15. The system of claim 13, wherein a SIGNAL field indicates the lengthof the cyclic prefix.
 16. The system of claim 13, wherein thetransceiver selects the length of the cyclic prefix.
 17. The system ofclaim 13, wherein a second transceiver selects the length of the cyclicprefix.
 18. The system of claim 13, wherein the transceiver is awireless 802.11a or a 802.11g transceiver.
 19. In an OFDM transceiver, amethod comprising: transmitting or receiving, over a first communicationchannel using a first cyclic prefix length, a first plurality of OFDMsymbols that carry data bytes; and changing from a first data rate to asecond data rate by transmitting or receiving, over a secondcommunication channel using a second cyclic prefix length, a secondplurality of OFDM symbols that carry data bytes, wherein: the firstcyclic prefix length is shorter than the second cyclic prefix length,the first communication channel has a shorter impulse response lengththan the second communication channel, the first cyclic prefix length isbased on the length of impulse response of the first communicationchannel, and the second cyclic prefix length is based on the length ofthe impulse response of the second communication channel.
 20. The methodof claim 19, wherein a message or a management frame indicates thelength of the cyclic prefix.
 21. The method of claim 19, wherein aSIGNAL field indicates the length of the cyclic prefix.
 22. The methodof claim 19, further comprising selecting, by the transceiver, thelength of the cyclic prefix.
 23. The method of claim 19, furthercomprising selecting, by a second transceiver, the length of the cyclicprefix.
 24. The method of claim 19, wherein the transceiver is awireless 802.11a or a 802.11g transceiver.